1. Field of the Invention
The present general inventive concept relates to a method of generating a halftone screen in an image forming device and a system thereof, and more particularly, to a method of generating a halftone screen to improve a printing quality in an image forming device, and a system thereof.
2. Description of the Related Art
Generally, printing devices have two brightness levels according to whether dots are printed, which is different from multi-level image devices. A method of printing a multi-level input image using a binary printing device is called halftoning.
In other words, an image having 256 brightness levels from 0 to 255 is generally called a continuous gray-level image, and a method of expressing or generating the continuous gray-level image in a binary output device (e.g., a printing device) using only 0 (i.e., black) and 255 (i.e., white) is called halftoning. An image generated based on a halftoning method is referred to as a binary image.
The halftoning method includes a screening method, an error spreading (diffusion) method to halftone through optimization. The screening method is a method of performing binarization by comparing a gray-level value of a pixel to be binarized with a predetermined threshold, which is part of a threshold array. The error spreading method is a method of taking an error occurring during binarization into consideration by spreading the error on surrounding pixels to be binarized based on a predetermined kernel value at a predetermined rate.
Although the screening method is a faster method than the error spreading method, the screening method has an inferior image quality at a low definition level. Since the error spreading method is not suitable for a laser printing device with irregular dot positions and sizes, the screening method is widely adopted by laser printing devices.
Screens are divided into an amplitude modulated (AM) screen and a frequency modulated (FM) screen according to how dots are arrayed. Since the AM screen is output with clusters of dots, the AM screen can be output more stably compared to the FM screen.
For this reason, the laser printing devices use the AM screen. The AM screens are classified into one of an AM ordered screen and an AM stochastic screen according to how the cluster dots are arrayed.
An output image binarized by using an AM ordered screen has a periodic cluster dot array or a periodic halftone dot array. However, an output image binarized by using an AM stochastic screen does not have the periodic cluster dot array.
The image binarized by using the AM ordered screen may have an unpleasant pattern due to the periodic cluster dot pattern. Particularly, when an input image has a periodic pattern, the output image has a subject moiré pattern having a periodic band in a predetermined direction.
In order to solve the above-described problems, a method of generating a screen which does not to have the periodic cluster dot array has been suggested. The conventional screen-generating method forms dot clusters from halftone dots using a spatial filter (i.e., an evaluation function).
FIG. 1 illustrates a conventional AM stochastic screen generating method. Generally, a screen can be generated by two methods which are illustrated in FIG. 1. One of the two methods is a direct dot growing method 10 using a spatial filter on an initial dot distribution and the other one of the two methods is a swapping growing method 20 using an initial binary pattern.
In the direct dot growing method 10, one arbitrary dot is selected as an initial dot and then a continuous dot order is determined based on the spatial filter. A multi-level input is converted into an output tone level based on the number of dots, so that a light gray-level range has a small number of dots while a shadow range has a large number of dots. Here, the number of dots increases as the output tone level varies from a light gray-level to a dark gray-level. The increase in the number of dots is called growing, and a gradual increase of the number of black dots is called order. Here, the order is determined according to a position having a minimum value after a mask operation using the spatial filter.
FIG. 2 illustrates a conventional dot order determining method. A predetermined dot distribution 30 has dots (pixels) grown according to a predetermined order of 0 to 14. A next dot order is the fifteenth order, and a spatial filter 40 is used to determine a position ‘A’ having a minimum value calculated by performing convolution on the predetermined dot distribution 30 by and using the spatial filter 40. The positions of the dots 1 to 15 are determined based on minimum values.
The following Equation 1 illustrates the above-described order determining method.cos t(i,j)=filter(i,j)**dot(i,j)  Equation 1where filter (j,j) denotes the spatial filter, dot(i,j) denotes a dot distribution, and ** denotes a circular convolution.
Dots with a determined order have a ‘1 (on)’ value, whereas dots without a determined order have a ‘0 (off)’ value. The mask operation is performed until determining positions of all dots that have a ‘1’ value. In short, when horizontal and vertical sizes of a screen are M and N, respectively, the dot order may have a value from ‘0’ to ‘M*N−1’. The following Equation 2 represents the above-described spatial filter.
                              filter          ⁢                                          ⁢                      (                          i              ,              j                        )                          =                              ⅇ                          -                                                                    ⅈ                    2                                    +                                      j                    2                                                                    2                  ⁢                                      σ                    1                    2                                                                                -                      ⅇ                          -                                                                    ⅈ                    2                                    +                                      j                    2                                                                    2                  ⁢                                      σ                    2                    2                                                                                                          Equation        ⁢                                  ⁢        2            
The Equation 2 uses a difference between two Gaussian functions, i and j are coordinates of a dot and, herein, a standard deviation ‘σ1’ should be always larger than a standard deviation ‘σ2.’ However, in the direct dot growing method, the dot distribution is not uniform in a highlight range. For this reason, the mask operation is carried out after the swapping growing method 20 using the initial binary pattern of FIG. 1 in the AM stochastic screen generating method.
In this case, after a predetermined number of dots that represent a particular gray-level are initially distributed arbitrarily, the initial dot distribution is rearrayed by using the spatial filter. A rearray operation is as follows.
First, a value of a cost function for an initial dot distribution is calculated and then the initial dot distribution is rearrayed. Then, a value of the cost function for the rearrayed dot distribution is calculated. The dot distribution with a smaller cost value between the initial distribution and the rearrayed distribution is stored. The above-described operation is repeated until the value of the cost function converges to a predetermined value. A final dot distribution is defined as a uniform binary pattern. The operation of rearraying the dots is called a swapping operation, and the numbers of black and white dots before and after the swapping operation should be the same.
When a uniform binary pattern is completed at a predetermined gray-level, the mask operation is performed by using the same spatial filter. For a gray-level lighter than the predetermined gray-level, dots are removed one by one and, for a gray-level darker than the predetermined gray-level, dots are added one by one. In an image binarized by using the above-described AM screen generating method, an undesirable circular pattern disappears compared to when using the AM ordered screen generating method.
However, although in the above-described conventional method a main distance of a cluster of dots is adjusted by tuning the values ‘σ1’ and ‘σ2’, the conventional halftoning method does not provide any basis for a proper controlling method of the main distance. In short, the conventional method has a problem in that it is hard to generate a screen such that the cluster of dots have a particular main distance.